Genomic analyses uncover many advanced generation hybrids.
In 1952, Thomas Howell published an extensive monograph on the Yellow-Bellied Sapsucker (Sphyrapicus varius) which then comprised four subspecies: varius, nuchalis, daggetti, and ruber. Based on extensive field observations, he attempted to figure out how often these subspecies hybridize. He came to the following conclusions:
Interbreeding between the races where their ranges meet is variable. It is apparently free between ruber and daggetti, moderate between daggetti and nuchalis and between varius and nuchalis, and rare or absent between ruber and nuchalis and between ruber and varius.
In other words, all subspecies seem to interbreed with one another (albeit at different frequencies). Over time, the taxonomy of these woodpeckers has changed. Ornithologists now recognize three distinct species:
- Yellow-Bellied Sapsucker (S. varius)
- Red-naped Sapsucker (S. nuchalis)
- Red-breasted Sapsucker (S. ruber, with subspecies ruber and daggetti)
These classificatory changes have provided some clarity, but the three species still interbreed in several hybrid zones. While these woodpeckers might give taxonomists a headache, they provide exciting opportunities for evolutionary biologists. Previous studies have already described genetic patterns in the hybrid zones between Red-naped and Red-Breasted Sapsucker, and between Red-breasted and Yellow-bellied Sapsucker. The third combination – Red-naped and Yellow-bellied Sapsucker – remained to be characterized with genetic data. Luckily, a recent study in the Journal of Avian Biology filled this knowledge gap.
Howell (1952) reported moderate hybridization between Red-naped and Yellow-bellied Sapsucker. Is this also reflected in the genetic make-up of these species? Using a set of three traditional markers and a more extensive genomic dataset, Libby Natola and her colleagues explored hybridization dynamics in the Rocky Mountains. The analyses revealed that most birds within the hybrid zone were genetically admixed: 89% based on traditional markers and 52% based on the genomic data. These patterns highlight that traditional markers, such as nuclear genes or microsatellites, tend to overestimate hybridization rates (see also this study on Chukar and Red-legged Partridge). Genomic data provide a more reliable picture.
Next, the researchers performed a more detailed analysis of the admixed individuals. They used “hybrid triangles” to determine the frequency of first-generation hybrids and backcrosses in the hybrid zone. These triangles combine information from a hybrid index (i.e. genetic ancestry of an individual) and the level of heterozygosity to discriminate between different hybrid classes. In general, “pure” individuals are located in the lower corners, while first generation hybrids are at the top. The sides of the triangles indicate backcrosses. These analyses suggested that “the majority of admixed individuals appear to be advanced generation hybrids.”
Problems with Plumage
Interestingly, the genetic ancestry of these birds was not reflected in their morphology. The researchers used an extensive eight-point system to classify the sapsuckers into different phenotypic classes. However, nineteen birds had genetic ancestry values that did not follow the phenotypic classification. Plumage is thus not a reliable indicator to discriminate between “pure” individuals and several hybrid classes.
And the situation is even more complex than described here. While writing this blog post, another paper on these sapsuckers was published in the journal Molecular Ecology. Apparently, two hybrid zones have collided into a tri-species hybrid zone where all three species interact. The researchers reported that “Surveys of the area […] show that all three species are sympatric, and Genotyping-by-Sequencing identifies hybrids from each species pair and birds with ancestry from all three species.” I will try to cover this study in due time. Stay tuned for another layer of complexity!
Billerman, S. M., Cicero, C., Bowie, R. C., & Carling, M. D. (2019). Phenotypic and genetic introgression across a moving woodpecker hybrid zone. Molecular Ecology, 28:1692-1708.
Grossen, C., Seneviratne, S. S., Croll, D. & Irwin, D. E. (2016). Strong reproductive isolation and narrow genomic tracts of differentiation among three woodpecker species in secondary contact. Molecular Ecology 25:4247-4266.
Natola, L., Curtis, A., Hudon, J., & Burg, T. M. (2021). Introgression between Sphyrapicus nuchalis and S. varius sapsuckers in a hybrid zone in west‐central Alberta. Journal of Avian Biology, 52(8).
Natola, L., Seneviratne, S. S., & Irwin, D. (2022). Population genomics of an emergent tri‐species hybrid zone. Molecular Ecology.
Featured image: Red-naped Sapsucker (S. nuchalis) © Matt MacGillivray | Wikimedia Commons